1. Field of the Invention
This invention relates to fluoropolymer insulated wire secured by a hinge that can be folded repeatedly with the folding and unfolding of the hinge without the insulation on the wire cracking.
2. Description of Related Art
To enable wireless communication in laptop computers, such computers are equipped with multiple antenna wires. The antenna wires are each comprised of fine wire insulated with tetrafluoroethylene/-perfluoro(alkyl vinyl ether) copolymer, commonly known as PFA. The antenna wire is positioned within the screen display section (portion) of the computer and is soldered into a circuit board in the keyboard section (portion) of the computer. The antenna wire communicates from the screen display section to the keyboard section by passing through the hinge(s) that interconnects the two computer sections. The antenna wire must satisfy electrical requirements as well as use requirements. With respect to use requirements, the antenna wire must be able to withstand repeated folding as the laptop computer is closed and opened and have solder resistance. At least one laptop computer manufacturer tests the folding and unfolding of the antenna wire and has found that the PFA insulation withstands at least about 25,000 cycles before failure (cracking). According to ASTM D 4565-99 (reapproved 2004), the temperature of the solder pot used in soldering is about 320° C. and the solder time may take up to ten seconds and longer, depending on the bulk of the wire and the terminal to which the wire is being soldered. Soldering onto a terminal that already has soldered present lengthens the time to complete the solder attachment of the antenna wire. During exposure of the wire insulation adjacent to the solder site of the wire, the wire insulation must not melt and pull back from the solder site. Such shrink-back leaves exposed, uninsulated conductor, which can lead to electrical failure of the antenna wire. PFA satisfies the electrical requirement and has sufficient flex life to withstand the laptop use. PFA also has a very high melting temperature, of about 305° C., which is sufficient to resist shrink-back during soldering.
The problem is that the number of antenna wires in laptops is multiplying to keep pace with wireless requirements, and the cost of the PFA insulated wire to form the antenna wire is becoming excessive to the computer manufacturer. The manufacturer desires to use a less expensive antenna wire insulation that will satisfy both its electrical and use requirements. Tetrafluoroethylene/hexafluoropropylene(HFP) copolymer, commonly known as FEP, is less expensive than PFA and is known to satisfy the electrical requirements of the antenna use, but has a much lower melting temperature than PFA and lower flex life. Experiments with FEP in the course of the investigation leading to the present invention have found that FEP exhibits poor solder resistance, apparently arising from its much lower melting temperature, about 250-255° C., than the temperature of the molten solder. As the FEP insulation melts, it flows away from the heat source, i.e. pulls back from the heat source, leaving uninsulated, exposed wire, which is subject to electrical failure. These experiments have also found that as the polymer is polymerized to a lower molecular weight to enable a reasonably high extrusion wire coating rate, which is necessary to realize the economy of using FEP as the insulation for the antenna wire, the loss in flex life is drastic. Such FEP does not withstand the repeated folding of the laptop computer, i.e. insulation of this FEP cracks. Antenna wire insulated with this FEP has exhibited less than 10,000 cycles before the insulation cracks in the laptop computer manufacturer folding test.
The problem remains of how to provide the performance of the PFA-insulated antenna but at less cost.